Radio-frequency switches (RF-SW) configured to turn on and off a radio frequency (RF) are used for front ends of portable communication terminals such as mobile phones. In such radio-frequency switches, a low loss of a radio frequency passing therethrough is an important characteristic. For such a low loss, it is important to reduce a resistance (on-resistance) of an FET in an on state or a capacitance (off-capacitance) of the FET in an off state, i.e., to reduce the product (Ron*Coff) of the on-resistance and the off-capacitance.
The off-capacitance includes a component (an intrinsic component) generated in, for example but not limited to, a diffusion layer and a substrate, and a component (an extrinsic component) generated in, for example but not limited to, a gate electrode, a contact plug, and a wiring line on the contact plug. For example, in the field of ultra-small MOSFETs, it is proposed to have an air gap around a gate electrode to reduce a parasitic capacitance between the gate electrode and a contact plug, thereby reducing an extrinsic component (for example, refer to Patent Literature 1).